In this type of installation, when the regulating valve is in a position in which it is at least partly open, the fresh air which is derived from the fresh air delivery chamber is distributed between the first and second ducts. Because of the respective positionings, in relation to the mixing zone, of secondary ducts which carry the air from the air treatment unit itself (i.e. the unit that includes the fresh air delivery chamber, the first and second ducts and the mixing zone, and which normally also includes a fan), to the ventilating and foot warming vents in the cabin, the temperature of the treated air delivered through the ventilating air vents is not the same as the temperature of that which is delivered through the foot warming vents.
This type of configuration is commonly known as a bi-level arrangement. In a bi-level arrangement, the air delivered at the level of the feet of the occupants of the vehicle is generally hotter than that which is supplied through the air ventilating vents. When a user desires to alter the cabin temperature, he sets a new temperature which is taken into account by the processing means. The latter then, in particular, acts to adjust the positional setting of the regulating valve, so as to modify the distribution of the fresh air between the above mentioned first and second ducts.
This adjustment of the positional setting of the regulating valve generally involves a substantial change in the temperature difference between the treated air delivered to the foot warming outlet vents and the treated air delivered to the ventilating vents. This can result in a great deal of discomfort and inconvenience for the occupants of the vehicle, and in particular the driver.